Let it sink in for a minute how many times you hear or read something in one day.
Go to the supermarket and try to notice everytime a message tries to reach you. You'll probably also be amazed by the amount of fucks you don't give for those messages, except for the brands that you choose to be a fan of.
To determine your ideal customer is to determine how you're going to talk to them. In the midst of that storm of information, you want to find your customer who'll listen to your unique message. There are most certainly a number of brands that you like for some reason. Think about one, and then think about the language they use to approach you as a customer. Is it soft, happy, short but powerful? Does it make you feel beautiful, welcome, amused or in for a treat?
In order to determine your own tone of voice, first list the preferences of your ideal customer. What does he or she need in his or her life? For instance, if you're a beautician, you want them to feel beautiful and that they deserve to treat their body well. If you're a funny beautician, you can also throw in some humour. Your tone of voice should be clear, but also personal and not unnatural.
Try writing down a little tone of voice manifest and make a few posts as an exercise. Have a little trouble finding your tone of voice? I'd love to help out!